5312
Langmuir 1996, 12, 5312-5318
Ammonium Bis(ethylhexyl) Phosphate: A New Surfactant for Microemulsions David C. Steytler,* D. Lee Sargeant, Gabriel E. Welsh, and Brian H. Robinson School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
Richard K. Heenan ISIS, Rutherford Appleton Laboratory, Chilton, OXON OX11 OQX, U.K. Received May 24, 1996. In Final Form: July 3, 1996X The ammonium salt of the metal ion extractant bis(ethylhexyl) hydrogen phosphate (HDEHP) has been found to stabilize water-in-oil (w/o) microemulsions over a wide range of ω ()[H2O]/[NH4DEHP]) and temperature. Such extensive solubilization has not been observed for Gp I, II, or III metal salts of HDEHP. In the absence of added water, SANS measurements show the formation of rod-shaped reversed micelles which progressively decrease in length as water is added in the range ω < 5. At constant ω the rod length increases with surfactant concentration. For ω > 5 spherical droplet w/o microemulsions are formed with the droplet size increasing in proportion to ω. Analysis of SANS data gives the head group area of NH4DEHP in the surfactant layer of larger spherical microemulsion droplets as 63 ( 3 Å2.
1. Introduction Bis(ethylhexyl) hydrogen phosphate (HDEHP) is an extractant of metal ions used in liquid membrane extraction from water.1 Both the free acid and the di- and trivalent metal salts of HDEHP, Mn+(DEHP-)n, have low solubility in water and are located almost exclusively within the oil “liquid membrane” (LM) which separates the aqueous “feed” and “strip” phases. Although the kinetics and mechanism of such processes have been previously examined,2,3 until recently there has been relatively little attention paid to the state of aggregation of the carrier (HDEHP) and its salt derivative(s) in oil media. In a previous paper4 we reported a structural study of reversed micelles formed by di- and trivalent metal salts of HDEHP in the oil cyclohexane, Mn+(DEHP-)n, where Mn+ ) Ca2+, Co2+, Ni2+, Cu2+, Mn2+, Al3+, and Cr3+. The extent of water uptake in the systems studied as defined by ω ) [H2O]/[Mn+ (DEHP-)n] was found to be low such that only hydrated reversed micelles were formed. The results supported formation of rod-shaped reversed micelles by all divalent metal ion salts with the SANS I(Q) well-represented by a rod form-factor with radii in the range 7.5-9 Å. The length of the rod-shaped reversed micelles formed was strongly dependent upon the nature of the counterion with fitted values between ∼80 Å (Mn+ ) Ni2+) and ∼300 Å (Mn2+). In contrast, the trivalent metal ion salt Al(DEHP)3 is present in solution either as a monomer or as small spherical aggregates of low aggregation number. The chemical structure of the anion DEHP- (Figure 1) bears a strong resemblance to that of sodium bis(2ethylhexyl)sulfosuccinate (Aerosol OT or AOT). This surfactant is very efficient at stabilizing water-in-oil (w/ o) microemulsions which have been extensively studied in a range of hydrocarbon oil media.5 These systems are thermodynamically stable and may be considered to be a X
Abstract published in Advance ACS Abstracts, October 1, 1996.
(1) Danesi, P. R. Sep. Sci. Technol. 1984, 19, 857. (2) Danesi, P. R.; Chiarizia, R. In CRC Critical Reviews in Analytical Chemistry; Campbell, B., CRC Press: Boca Raton, FL, 1980; pp 1-126. (3) Hughes, M. A. Biswas, R. K. Hydrometallurgy 1991, 26, 281. (4) Steytler, D. C.; Jenta, T.; Robinson, B. H.; Eastoe, J.; Heenan, R. K. Langmuir 1996, 12, 1483-1489. (5) Tapas, K. D. Adv. Colloid Interface Sci. 1995, 59, 95-93.
S0743-7463(96)00509-4 CCC: $12.00
Figure 1. Molecular structure of the ammonium salt of HDEHP.
dispersion of essentially spherical water droplets with mean water core radius R h c given by6-8
R hc )
3Vwω NAvAs
(1)
where Vw ) the molar volume of water, ω ) [H2O]/ [surfactant]i, As ) the surface area of the surfactant at the water/surfactant interface, and [surfactant]i ) the concentration of surfactant in the system located at the interface. For small droplets (ω < 25) the extent of polydispersity is found to be small.6,7 At constant composition, AOTstabilized w/o microemulsions exhibit a region of stability between an upper (UTPB) and lower (LTPB) temperature phase boundary where the single-phase (1φ), isotropic L2 microemulsion becomes unstable, as shown in Figure 2 for microemulsions formed in n-heptane. The conditions that induce phase separation of the microemulsion and the processes which occur have been studied previously, and both the thermodynamic and kinetic driving forces for phase separation have been discussed in detail.9 At the LTPB the instability is believed to be due to curvature effects such that Winsor II systems10 are formed in the microemulsion phase with the droplet radius governed by a minimum energy condition for curvature of the surfactant layer often referred to as the “natural interfacial (6) Chen, S.-H. Annu. Rev. Phys. Chem. 1986, 37, 351-399. (7) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 985-1006. (8) Kotlarchyk, M.; Huang, J. S.; Chen, S.-H. J. Phys. Chem. 1985, 89, 4382. (9) (a) Eastoe, J.; Robinson, B. H.; Steytler, D. C. J. Chem. Soc., Faraday Trans. 1990, 86, 511-517. (b) Eastoe, J.; Robinson, B. H.; Steytler, D. C.; Young, W. K. J. Chem. Soc., Faraday Trans. 1990, 86, 2883-2889. (10) Winsor, P. A. Solvent Properties of Amphiphilic Compounds; Butterworths: London, 1954.
© 1996 American Chemical Society
Ammonium Bis(ethylhexyl) Phosphate
Langmuir, Vol. 12, No. 22, 1996 5313
phate (NH4DEHP). Unlike NaDEHP, or other group I metal salts, NH4DEHP is found to be a very effective surfactant for stabilizing w/o microemulsions, giving, for most oils, a more extended single-phase region than even AOT. Interestingly the improved efficiency of NH4+ compared with Na+ in stabilizing DEHP w/o microemulsions is not observed for AOT systems for which solubilization in the L2 region is found to be reduced (ωmax ∼ 14) for the ammonium salt NH4AOT in cyclohexane. 2. Experimental Section
Figure 2. Solubilization-temperature (ωmax-T) phase diagrams for NH4DEHP w/o microemulsions in n-heptane (s) and cyclohexane (- - -). Phase boundaries for the AOT-stabilized w/o microemulsion in n-heptane and also shown for comparison (‚‚‚). The details of phase separation at points A and B are given in the text. [AOT], [NH4DEHP] ) 0.1 mol dm-3.
curvature”. The instability at the UTPB is more complex in nature with increasing contributions from (i) attractive droplet interactions and (ii) a propensity to more positive interfacial curvature, i.e. ultimately formation of o/w microemulsions (Winsor I systems). The effectiveness of a particular surfactant in stabilizing w/o microemulsions may be gauged by the extent of the single-phase (1φ) region as measured by both the extent of water solubilization (ωmax) and the temperature range over which the microemulsion is stable. The formation of microemulsions by sodium bis(2ethylhexyl) phosphate (NaDEHP) has been examined by Harada et al.,11 who made a detailed study of the phase behavior of the pseudoternary system NaDEHP/H2O (NaCl)/n-heptane. The ternary mixture was found to form Winsor microemulsion systems10 and responded to changes in salt concentration in a manner entirely analogous to that observed for ternary mixtures of anionic surfactant(s), such as AOT, oil, and water. However, compared with AOT the extent of solubilization in NaDEHP-stabilized w/o microemulsion phases was found to be poor (ωmax < 5). Light-scattering and viscosity studies11 of dilute solutions of NaDEHP in oil media have revealed an unusual “anomalous” effect wherein the cmc is raised on addition of small amounts of water. Such behavior is apparently opposite to that observed for sulfosuccinate surfactants of similar structure, such as Aerosol OT (NaAOT) and the related nickel salt Ni(AOT)2, where trace amounts of water are found to promote aggregation, i.e. lower the cmc. At low water content NaDEHP solutions are also highly viscous due to the formation of extremely long, rod-shaped reversed micelles. On addition of water the length of the micelles decreases with a commensurate decline in both the solution viscosity and the radius of gyration of the micelles. An explanation of the effect of water has been proposed12 in terms of the influence on the ionic bonding in the core of the micelles. More recently the ternary system NaDEHP/water/n-heptane has been examined at higher surfactant concentrations both in the oil-continuous13 regime and at higher ω values where bicontinuous and o/w microemulsions14 are observed. In this paper we present the first study of w/o microemulsions formed by ammonium bis(2-ethylhexyl) phos(11) Shioi, A.; Harada, M.; Matsumoto, K. J. Phys. Chem. 1991, 95, 7495- 7502. (12) Yu, Z.-J.; Neuman, R. D. J. Am. Chem. Soc. 1994, 116, 40754076.
2.1. Preparation of NH4DEHP. HDEHP (97%, Aldrich) was initially purified according to the method of Partridge et al.15 The ammonium salt of the acid NH4DEHP was prepared by neutralization of the acid with ammonium hydroxide. Purified HDEHP was dissolved in ethanol (Aldrich); aqueous ammonium hydroxide (approximately 35% NH3, BDH) was then added to the stirred solution until the acid was completely neutralized (pH > 8). Excess ammonia and solvent were then removed using a rotary evaporator at 60 °C for 2 h to leave NH4DEHP, a white, waxy solid. The product was initially dried in a vacuum oven at room temperature for ∼48 h and further dried by storage over P2O5 in a vacuum desiccator until required. 2.2. Phase Diagrams. Binary ωmax-T phase diagrams showing the extent of the single-phase microemulsion region were determined by direct visual observation of the onset of turbidity/phase separation. Microemulsions with ω values in the range 5-100 were prepared ([NH4DEHP])0.1M) in sealed screw-top vials. The samples (2 mL) were then thermostated and allowed to equilibrate for 1 h at 0.0 ( 0.1 °C in a Grant LTD6 water bath. The samples were then carefully viewed to identify which were single phase. The process was then repeated at increasing temperatures up to the maximum temperature examined. 2.3. Polarizing Microscopy. Optical birefringence of liquidcrystalline structures was examined using an Olympus BH-2 optical microscope fitted with cross-polarizing lenses. Samples of NH4DEHP were mounted on a glass slide beneath a cover slip and then contacted with a small amount of water at the edge of the sample. Diffusion of water into the surfactant results in a gradient of surfactant concentration which is highest in the center of the sample and more dilute at the outer edge. Liquid crystal formation under the slip therefore occurs in stratified layers and can be identified by characteristic birefringence patterns when viewed under cross-polars. Photographs of the resulting liquidcrystalline phases were taken using an Olympus MD-OM2N camera. 2.4. Small-Angle Neutron Scattering (SANS). SANS measurements were performed on the LOQ spectrometer using the ISIS pulsed neutron source of the EPSRC Rutherford Appleton Laboratory, U.K. The magnitude of the momentum transfer vector Q is given by
Q)
4π θ sin λ 2
()
(2)
where λ is the incident wavelength (2.2-10.0 Å), determined by time-of-flight, and θ is the scattering angle. The intensity of neutrons was recorded on a position-sensitive 64 × 64 pixel 2-D detector at a fixed sample-to-detector position (4.3 m), providing an effective Q range from 0.008 to 0.22 Å-1 in a single measurement. The data were corrected for transmission and incoherent background scattering and were normalized to absolute scattering probabilities (cm-1) using standard procedures. Further details of technical and experimental aspects together with data reduction procedures are given elsewhere.16 Samples were contained in stoppered, matched Hellma quartz glass cells (1 or 2 mm path length) and thermostated at 25 ( (13) Kurumada, K.; Shioi, A.; Harada, M. J. Chem. Phys. 1994, 98, 12382-12389. (14) Yu, Z.-J.; Neuman, R. D. Langmuir 1995, 11, 1081-1086. (15) Partridge, J. A.; Jensen, R. C. J. Inorg. Nucl. Chem. 1969, 31, 2587. (16) Heenan, R. K.; King, S. M.; Osborn, R.; Stanley, H. B. Rutherford Appleton Laboratory Report RAL-89-128, 1989.
5314 Langmuir, Vol. 12, No. 22, 1996
Steytler et al.
0.1°C. Two contrast profiles were employed: (i) a droplet contrast, H2O/NH4DEHP(H)/C6D12, providing a measure of the mean overall droplet radius, R h h, i.e. water core plus surfactant layer, and (ii) a water core contrast, D2O/NH4DEHP(H)/C6H12, giving a measure of the mean core radius, R h c. For low ω systems scattering using the core contrast is extremely weak, and so only droplet contrast was used. Deuterated chemicals were cyclohexane-d12 (MSD Isotope, 98.5% D atom) and deuterium oxide (C/D/N Isotopes Inc., 99.9% D atom). 2.5. Viscosity Measurements. The viscosities of reversed micelles and w/o microemulsions formed by NH4DEHP were measured using Ubbelohde viscometers (Schott-Gera¨te) immersed in a thermostatic bath (Townson and Mercer) at 25 ( 0.1 °C. The flow times of the solvent and solutions were corrected for density.
(ηred)φf0 )
3. Theory 3.1. SANS. For small particles (or micelles) of volume Vp present at number density np the normalized SANS intensity I(Q) (cm-1) may be written 2
2
I(Q) ) npVp ∆F P(Q) S(Q)
(3)
where ∆F is the difference in the scattering length densities of the dispersed phase (e.g. particles, Fp) and the solvent medium (cyclohexane, Fs). P(Q) is the single-particle form factor, which describes the angular distribution of the scattering that arises from the size and shape of the particle. Expressions for P(Q) representing various particle shapes, such as spheres, rods, disks, ellipsoids, etc., can be used to model SANS data17 in order to determine particle shape and size. S(Q) is the structure factor which arises from spatial correlations between particles. For dilute reversed micelles and microemulsion droplets that are far-removed from phase boundaries, i.e. in the absence of attractive interactions, S(Q) f 1.0. Under these conditions I(Q) is a direct measure of P(Q); i.e.,
I(Q) ) npVp2∆F2P(Q)
(4)
For reversed micelle systems at fixed volume fraction the S(Q) contribution will become increasingly significant as the aggregation number (micelle size) decreases. Theoretical models are available for S(Q) when micelles are spherical, but as yet no simple formalism exists to account for interactions between anisotropic rod- or diskshaped structures. However, since the aggregation number is then appreciable, the S(Q) contribution is less significant than for spherical micelles formed at the same surfactant concentration. Since P(Q) ) 1 when Q ) 0 and the I(Q) data are fitted in absolute units the value of the scale factor is a selfconsistency check on the model, since both npVp ()φ) and ∆F2 are known. Our fitting program allows us to examine a wide range of models from which physically unrealistic solutions can be eliminated using, in part, the scale factor criteria.17 According to Porod,18 for a sharp (stepwise) interface the asymptotic value of I(Q) should be
S I(Q) ) 2π∆F2 Q-4 V
solutions in oil media. When the core contrast is used for microemulsions, this gives an independent measure of the surface area per surfactant molecule occupied at the water/surfactant interface of the droplets (As). 3.2. Viscosity. The viscosity of a solution of macromolecules, micelles, or particles of colloidal dimensions is determined in the dilute regime by particle shape, but as concentration (φ) is increased, interparticle interactions also contribute to the measured viscosity. For spheres, in the condition of infinite dilution (φ f 0), Einstein19 showed that the reduced viscosity, ηred, approaches a limiting value of 2.5:
(5)
where S/V is the total interfacial area per unit volume of the sample. Provided that the contrast ∆F is known, eq 5 can be used to estimate the value of the parameter S/V of the reversed micelles or microemulsions formed by surfactant (17) Heenan, R. K. FISH Data Analysis Program, Rutherford Appleton Laboratory Report, RAL-89-129, 1989. (18) Porod, G. Kolloid Z. 1951, 124, 82.
{ } ηsp φ
φf0
) 2.5
where ηsp is the specific viscosity given by
ηsp )
{
}
ηsoln - ηsolv ηsolv
(6)
with ηsoln ) the solution viscosity at the volume fraction φ and ηsolv ) the solvent viscosity. The reduced viscosity at infinite dilution is often referred to as the intrinsic viscosity , [η], and can be used to give a measure of the axial ratio J of anisotropic, ellipsoidal particles:20
[η] ) 2.5 + 0.4075(J - 1)1.508
(7)
where J ) r1/r2, r1 ) the major axis of revolution, and r2 ) the minor axis of revolution. In principle eq 7 can be applied to viscosity data to extract information concerning the length of long rodshaped particles provided an estimate, or independent measurement, of the radius is available. 4. Results/Discussion 4.1. Phase Behavior. Binary solubilization-temperature (ωmax vs T) phase diagrams showing the extent of stable, single-phase (1φ), w/o microemulsions formed by NH4DEHP in n-heptane and cyclohexane are shown in Figure 2. The 1φ region in n-heptane is seen to be appreciably more extensive than that for microemulsions stabilized by AOT which exhibit a characteristic “funnel” of stability at high ω values.21 The nature of phase separation at the phase boundaries was examined in n-heptane for the two paths indicated in Figure 2. It was observed that path A (T ) 25 °C) resulted in phase separation reminiscent of “critical” instability22 with the microemulsion separating into two oil-continuous phases of approximately equal volume. Conductivity measurements confirmed the role of attractive interactions in the phase transition and showed a steep increase as the phase boundary was approached characteristic of extensive droplet clustering. Although not always observed, this type of instability is expected, since increasingly attractive interactions between droplets are predicted with increasing droplet size due to the greater “overlap volume” between surfactant layers of droplets in contact.23 Instability at the UTPB (path B, ω (19) Einstein, A. Ann. Phys. 1906, 19, 289; 1911, 34, 591. (20) Frish, H. L.; Shima, R. In Rheology; Eirich, F. R., Ed.; Academic Press: New York, 1956; Vol. 1. (21) Howe, A. M. Ph.D. Thesis, University of Kent at Canterbury, 1986. (22) Kotlarchyk, M.; Chen, S.-H.; Huang, J. S.; Kim, M. W. Phys. Rev. A 1984, 29, 2054-2069. (23) Peck, D. G.; Johnston, K. P. J. Phys. Chem. 1991, 95, 9549 -9556.
Ammonium Bis(ethylhexyl) Phosphate
Langmuir, Vol. 12, No. 22, 1996 5315
Figure 3. Solubilization-temperature (ωmax-T) phase diagram for NH4DEHP-stabilized w/o microemulsions in n-nonane (s) and n-pentane (‚‚‚). [NH4DEHP] ) 0.1 mol dm-3.
) 40) gives rise to a Winsor I type phase separation with a surfactant-rich aqueous phase separated from an upper oil phase. The effect of oil type is illustrated by the binary phase diagrams shown in Figure 3 for n-nonane and n-pentane. In common with AOT-stabilized microemulsions, the range of stability of the single phase microemulsion moves to lower temperatures as the oil molecular weight increases. This observation is in agreement with the original R-Theory of Winsor, which, based on arguments of interfacial curvature, provides a powerful predictive model for rationalizing the response of microemulsions to system parameters (oil carbon number (n), surfactant structure, temperature, ionic strength of the aqueous phase, etc.).24 The extent of solubilization is also progressively reduced as the carbon number of the alkane is increased. This response is also shown by AOT-stabilized microemulsions and is due to a decline in surfactant/oil compatibility with increasing n which drives the increasingly stronger attractive interaction between the droplets.22,23 For NH4DEHP the most extended L2 phase is obtained with n-pentane, suggesting an optimum alkane carbon number for solubilization of e5 for this surfactant. Surfactants based on HDEHP, with a pKa ∼ 5, are inherently unstable under weakly acid conditions. Phase separation of microemulsions formed by NH4DEHP can therefore be easily induced at low hydrogen ion concentration. An interesting illustration of this behavior is observed if CO2 is bubbled through a NH4DEHP w/o microemulsion, when, after ∼30 s, phase separation occurs as CO2 dissolves in the water droplets, forming carbonic acid, which partially protonates the surfactant. The phase separation is characteristic of a Winsor II type transition with a water phase separating from the oil phase containing a mixture of HDEHP and NH4DEHP. On stopping the CO2 stream, the microemulsion will reform on gentle agitation as the dissolved CO2 slowly escapes and the equilibrium with atmospheric levels of CO2 is re-established, causing the pH to rise. This unusual response provides a facile means of reversibly separating the aqueous phase of the microemulsion and may well find application in microemulsion-based separation processes25 and synthesis. One limitation of the surfactant AOT is the accelerated rate of hydrolysis of the ester groups of the molecule under (24) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems: Formation, Solvency, and Physical Properties; Surfactant Science Series Vol. 30; Marcel Dekker, Inc.: New York, 1988. (25) Hatton, T. A. In Ordered Media in Chemical Separations; Hinze, W. L., Armstrong, D. W., Eds.; ACS Symposium Series 342; American Chemical Society: Washington, DC, 1987; p 170.
Figure 4. SANS data (I(Q) vs Q) for rod-shaped reversed micelles and microemulsions formed by NH4DEHP at low ω values in cyclohexane-d12. The data sets have been displaced (by 3 cm-1) for clarity. Also shown as solid lines are the fits to P(Q) for monodisperse rods ([NH4DEHP] ) 0.088 mol dm-3; T ) 25 °C).
Figure 5. Dependence of rod lengths (obtained from SANS measurements) on surfactant concentration for hydrated reversed micelles (ω ) 2) of NH4DEHP in cyclohexane-d12 (T ) 25 °C).
alkaline conditions, which has restricted the range of pH for reactions carried out in AOT-stabilized w/o microemulsions. In an attempt to solve this problem, more robust, “nitrogen analogues” of AOT have recently been synthesized with an amide linkage replacing the sulfosuccinate ester groups.26 The chemical stability of these “new” surfactants was found to be excellent, but both their solubility and solubilizing capacity for water in n-alkane oils were disappointingly low. Although the high pKa of HDEHP makes NH4DEHP inherently unstable under mildly acidic conditions, the phosphate ester is very stable under alkaline conditions. NH4DEHP therefore provides (26) Leydet, A.; Boyer, B.; Lamaty, G.; Roque, J. P.; Catlin, K.; Menger, F. M. Langmuir 1994, 10, 1000-1002.
5316 Langmuir, Vol. 12, No. 22, 1996
Steytler et al.
Figure 8. Dimensions of reversed micelles and w/o microemulsions formed by NH4DEHP in cyclohexane showing the transition between cylindrical (length L; radius rh) and spherical (radius R h h) structures (T ) 25 °C).
Figure 6. SANS data (I(Q) vs Q) for spherical w/o microemulsion droplets formed by NH4DEHP for ω > 5 in cyclohexane-d12). The data sets have been normalized such that I(Qf0) ) 1 cm-1 and are displaced (by 1 cm-1) for clarity. Best fits to the data using the Shultz polydisperse sphere model (eq 5) are also shown as solid lines ([NH4DEHP] ) 0.088 mol dm-3; T ) 25 °C).
Figure 9. Porod plot of the SANS data for spherical w/o microemulsion droplets: ω ) 30 (O), 35 (4), and 40 (3) ([NH4DEHP] ) 0.059 mol dm-3; T ) 25 °C).
Figure 7. Dependence of mean droplet radius R h h (4) and water core radius R h c (O) on ω for w/o microemulsions stabilized by NH4DEHP in cyclohexane ([NH4DEHP] ) 0.088 mol dm-3; T ) 25 °C).
a viable alternative to AOT for applications of w/o microemulsions as reaction media at elevated pH. 4.2. SANS Measurements. The SANS results clearly show a transition from rod-shaped hydrated reversed micelles, formed at low ω values, to spherical w/o microemulsions at ω > 4. SANS data for the low ω systems was well-represented by a monodisperse rod form-factor with length L and radius rh as parameters (Figure 4). The results demonstrate a decrease in the rod length as the surfactant becomes more hydrated. However, it was not possible to determine the rod length for ω < 2, since the experimental data do not extend to low enough Q to define the Q range in which the form-factor is sensitive to L. For the ω ) 0 and 1 data sets
Figure 10. Dependence of reduced viscosity on concentration for micelles and w/o microemulsions formed by NH4DEHP in cyclohexane (T ) 25 °C).
all values of L above ∼300 Å were therefore found to fit the data equally well, so that SANS measurements provide only a lower limit for L in this low-ω regime. The rod radius, rh, is, however, a sensitive parameter in the data analysis for all rod-shaped systems examined and increases in a predictable fashion with ω.
Ammonium Bis(ethylhexyl) Phosphate
Langmuir, Vol. 12, No. 22, 1996 5317
Figure 11. Polarizing microscope image of liquid crystal phases formed by neat NH4DEHP in contact with water.
To determine the effect of concentration on rod length, SANS experiments were also performed on a dilution series for a ω ) 2 system in the range 0.25-5.0% w/v of the dispersed phase (surfactant plus water). The results (Figure 5) clearly show a cooperative effect with the rod length progressively increasing with concentration from 48 Å at 0.25% w/v to 93 Å at 5.0% w/v. Although predicted by the mean-field theory,27 such behavior is not commonly observed for reversed micelles formed by surfactants in oil media and has not been widely reported. Above ω ) 5 the scattering is consistent with spherical droplets, and representative fits to a spherical form-factor are shown in Figure 6 for the droplet contrast profile in cyclohexane. As previously found for w/o microemulsions stabilized by AOT, the droplets exhibit a small degree of polydispersity which could be accounted for using a Schultz h i ) 0.2. distribution function28 with width parameter σ/R h h, The radii obtained for the water core, R h c, and droplet, R for the two contrast situations are given in Figure 7 as a function of ω. The linear dependence of radius on ω is in accord with eq 1, indicating that As is, to a first approximation, independent of droplet size. The value h c data of Figure 7 is 64 ( 3 Å2, obtained for As from the R which is essentially the same as that of AOT. The difference between the radii measured using the two h c) gives ∼5 Å as a measure of the effective contrasts (R hh-R thickness, ts, of the surfactant layer. This is shorter than expected from the fully extended, “all trans”, surfactant structure and suggests a degree of penetration of water and/or oil. An alternative interpretation can be made in terms of splaying of the 2-ethylhexyl hydrocarbon chains into a tilted configuration. Measurements of surfactant layer thickness in DDAB-stabilized w/o microemulsions have also given values for ts significantly lower than the fully extended chain length of the hydrophobe.29 The (27) Cates, M. E. J. Phys. (Paris) 1988, 49, 1593. (28) Kotlarchyk, M.; Chen, S. H. J. Chem. Phys. 1983, 79, 2461.
results of the SANS measurements are summarized in Figure 8, which shows the transition between rod-shaped and spherical microemulsions at a ω value of ∼4. SANS measurements were also made using the core contrast condition on w/o microemulsions stabilized by NH4DEHP in n-heptane for ω ) 10-40. The scattering for this system required a higher value for the width distribution of polydispersity (σ ) 0.3) in order to fit the data effectively. A linear dependence of droplet radius on ω was also obtained in n-heptane, yielding the same value of As for the surfactant as in cyclohexane. A Porod analysis of the core-contrast SANS data was made for the “larger” microemulsion droplets (ω > 25), for which representative data are shown in Figure 9. This droplet size range gives optimal statistical resolution of the Porod region for the SANS instrument employed (LOQ). The graph shows a well-resolved, common asymptote for all three samples. Application of eq 5 with ∆F ) 6.9 × 1010 cm-2 gave As as 64 ( 3 Å2 for both n-heptane and cyclohexane, which is in good agreement with the value derived by an alternative analysis (eq 1) using the gradient of the data shown in Figure 7. 4.3. Viscosity Measurements. Viscosity data, displayed as ηred vs φ, are shown in Figure 10 for reversed micelles and w/o microemulsions formed by NH4DEHP. The response of the system to water closely mirrors that of the SANS data with the intrinsic viscosity of the low ω value systems decreasing with increasing ω as the rod length decreases. This general pattern of behavior is also observed for NaDEHP, for which the magnitude of the viscosity at low ω values is considerably higher.30 In principle, eq 7 can be applied to viscosity data to extract information concerning the length of rod-shaped micelles, provided an estimate, or independent measurement, of (29) Eastoe, J.; Dong, J.; Heenan, R. K.; Steytler, D. C. J. Chem. Soc., Faraday Trans. 1996, 92, 65-72, (30) Yu, Z.-J.; Neuman, R. D. Langmuir 1994, 8, 2553-2558.
5318 Langmuir, Vol. 12, No. 22, 1996
the radius is available. However, in applying eq 7, it is assumed that the micelle size and shape are independent of surfactant concentration over the concentration range in which the extrapolation to infinite dilution is conducted. Since SANS measurements clearly demonstrate that the rod length is not independent of concentration, a quantitative analysis of viscosity data was not attempted. Above ω ) 4 the viscosity data are characteristic of spherical droplets, giving intrinsic viscosities close to 2.5, in good agreement with the Einstein prediction for hard spheres. 4.4. Polarizing Microscopy. Figure 11 shows neat NH4DEHP in contact with water, viewed between crossed polars on a polarizing microscope. The photograph reveals the liquid crystal phases formed in surfactant/water mixtures across a continuous gradient of composition, with the surfactant concentration increasing from the bottom right to the top left of the picture. In this direction a transition from L1 (isotopic) f lamellar, LR f reversed cubic I2 f reversed hexagonal H2 is indicated. The H2 f I2 transition at low water content mirrors the rod f sphere transition in the microemulsions formed in the dilute L2 phase with increasing ω. The affect of ω on the shape of reversed micelles and microemulsions formed in the L2 phase is therefore mirrored also in the liquid crystal phases formed in binary surfactant/water mixtures. 5. Conclusions In the absence of water NH4DEHP forms extended rodshaped reversed micelles in oil media of length L > 300 Å, giving rise to solutions of high viscosity. As water is
Steytler et al.
added the micelles become progressively shorter in length until spherical droplets are formed at ω ∼ 4. With further addition of water the droplets grow in size with the core radius Rc increasing in proportion to the water content ω. Binary ωmax-T phase diagrams show NH4DEHP to be a very effective surfactant for stabilizing w/o microemulsions over a wide range of droplet size (ωmax < 100) and temperature (0-90 °C). The single-phase microemulsion region on such phase diagrams generally shows either an extended thermal stability with low solubilization (e.g. DDAB ωmax ∼ 12) or an appreciable water solubilization over a limited temperature range which becomes narrower with increasing ω (e.g. AOT, nonionics CnEm). NH4DEHP therefore has advantages in applications where high levels of water solubilization are required over a wide temperature range. Unlike NaAOT, which is readily hydrolyzed, NH4DEHP is chemically stable under basic conditions and may be a better choice of surfactant in applications of w/o microemulsions at elevated pH. Conversely, the high pKa of the parent acid, HDEHP, confers instability under mild acid conditions, resulting in separation of the dispersed aqueous phase. However the reversibility of this instability provides a novel route for facile separation of solubilized components in the microemulsion droplets and may prove a valuable asset in microemulsion-facilitated separation processes. LA9605091
